Known examples of antenna devices which resonate at two resonant frequencies include the planar inverted-F antenna disclosed in Japanese Pat. Pub. No. 61-41205 (Pat. Appl. No.59-162690) and microstrip antennas presented in "Handbook of Microstrip Antennas" by J. R. James and P. S. Hall.
FIG. 1 is a perspective view showing the construction of the planar inverted-F antenna disclosed in the above-mentioned application. This prior art example has a first planar radiation element 21 and a second planar radiation element 22, and these are arranged parallel to ground plane 23. The two planar radiation elements 21 and 22 are mutually connected by stub 24, and first planar radiation element 21 and ground plane 23 are connected by stub 25. The non-grounded conductor of feed line 26 is connected to planar radiation element 21 at contact point 27, while the grounded conductor of feed line 26 is connected to ground plane 23. The dimensions L.sub.1 .times.L.sub.2 of planar radiation element 21 differ from the dimensions L.sub.3 .times.L.sub.4 of planar radiation element 22, which means that they resonate at different resonant frequencies to give a double resonance. In other words, the planar inverted-F antenna constituted by planar radiation element 21 and the planar inverted-F antenna carried on top of it resonate independently, and are fed by a single feed line 26.
FIGS. 2-4 show examples of three cross-sectional structures of microstrip antennas. In these antennas, first planar radiation element 31 and second planar radiation element 32 are again arranged parallel to ground plane 33, but two feed lines 34 and 35 are connected to these (in the example given in FIG. 4, only feed line 34 is connected). In these cases as well, the size and structure of the two planar radiation elements 31 and 32 are different, and they resonate independently to give a double resonance.
Consequently, the thickness h.sub.2 of a conventional double-resonance planar inverted-F antenna has to be approximately twice the thickness h.sub.1 of a single planar inverted-F antenna. The disadvantage of the prior art has therefore been that an antenna has to have a larger capacity and a more complicated structure in order to obtain double resonance characteristics.
Conventional double-resonance microstrip antennas have the advantage that the two frequencies can be selected relatively freely, but because structurally they are basically two antennas on top of one another, the disadvantage has again been that the antenna volume is larger and its structure more complicated. A further disadvantage of multiresonant microstrip antennas of the basic type has been their lack of resonance below the first mode resonant frequency.
The purpose of this invention is to solve such problems and to provide an antenna device which, although small and simple in construction, has double resonance characteristics.